The retention and movement of water in soils, its uptake and translocation in plants and its loss to the atmosphere are all energy-related phenomena. All substances including water have a tendency to move or change from a state of higher to one of lower energy. Water movements in soils will generally be from a zone where the energy level of the water is high (wet soil) to one where the energy level is low (dry soil). If the pertinent energy levels at various points in the soil are known, the direction of water movement can be predicted.
Three important forces affect the energy level of soil water.
First adhesion, or the attraction of the soil solids (matrix) for water, provides a matric force (responsible for the phenomenon of adsorption and capillarity) that markedly reduces the energy state of the adsorbed water molecules. To a lesser extent the forces of cohesion which describes the attraction of water molecules to each other lowers the energy state. Together these forces produce a suction force within soil.
Second, the attraction of ions and other solutes towards water, result in osmotic forces, that tend to reduce the energy level in the soil solution. Osmotic movement of pure water across a semipermeable membrane into a soil solution is evidence of the lower energy state of the soil solution.
The third major force acting on soil water is gravitational force, which tends to pull water downward. The energy level of water at a given elevation in the soil profile is thus higher than that of water at a lower elevation. This difference in energy level causes water to flow downward.
Energy levels of soil water are usefully compared with that of a sample of pure water outside the soil in an environment that is maintained at standard pressure, temperature, and elevation levels. The difference in energy levels between the pure water and that of soil water is termed soil water potential.
Soil water (matric) potential is always negative, because it requires a force to draw water from the soil matrix implying that the water is moving from a higher energy state towards the lower energy state of pure water.
Every soil type has the ability to retain a specific volume of water due to its texture and pore volume or soil porosity (empty spaces between soil colloids). The matric force or the matric potential (ψ) which results from the phenomena of adhesion (or adsorption) and of capillarity, influences soil moisture retention or the total amount of stored soil water as well as soil water movement.
The volume of water held in soil that has been watered and has become fully drained (the point in time, where maximum amount of water is held against gravity) is called the field capacity of that soil type.
Therefore, a fine textured soil type such as clay contains by its physical nature, numerous small pores and capillary channels in which water can be retained against the pull of gravity because of matric forces exerted by the soil on soil water. Contrastingly, coarse, sandy soil has fewer, but larger pores in which lower matric forces contribute to lower soil water storage potential at field capacity.
The size and amount of pores in the soil type and a given soil water content at a point in time, dictates the matric force by which water is bound to the soil particles in that type of soil. This matric force is equivalent to the negative tension (suction−matric potential), that plant roots have to overcome to remove water from the soil for uptake.
The permanent wilting point is defined as the water content at which plants can no longer extract soil water at a rate sufficient to meet physiological demand. The physical demand of the plant is determined by the loss of water to the atmosphere (evapotranspiration) and at wilting point plants wilt and die. The permanent wilting point is generally reached at a matric potential of −15 bars.
A primary practical use of field capacity and wilting point concepts is the determination of a plant available soil water range (PASW). Soil water storage available for plant use is generally calculated as being between field capacity and permanent wilting point.
Different soils can have greatly differing wetness versus matric potential (tension or suction) relationships (e.g., a sandy soil may retain less than 5% water content at −15 bar matric potential, whereas a clayey soil may retain three times as much). Clearly, the more compacted a soil compound is, the less capacity there exists to retain soil moisture because of reduced pore volume or porosity with the converse also being true. Therefore, different absolute amounts of water are stored in different soil textures.
The concept of plant available soil water storage is an important factor in the determination of irrigation amounts for a cropped field or other soil-plant system. For practical purposes, irrigation amounts in excess of field capacity are lost. This is so because the excess soil water percolates away, and, this should be avoided in the interest of water resource conservation. There is also a potential for the leaching of beneficial soluble salts and chemicals from the zone within which the crop can draw soil water containing those salts and chemicals.
The relation between the volumetric soil water content (%) (which is the percentage amount of soil water retained by the soil on a volume basis) and soil water tension (suction, matric potential) is referred to as the water retention characteristic and this relation can be described mathematically using a soil water release curve.Gravimetric soil water content W=(Mw−Md)/Md(g g−1)WhereMw=wet mass of soil core (g)Md=dry mass or soil core (g)Bulk density ρ=Md/V cm−3 Volumetric water content θv=Wρ1
A tensiometer is but one of a number of devices used to measure the force by which the soil retains the water which in turn is related to how difficult the root system of a plant finds it to extract water from the soil. Although there are many other apparatus, tensiometers are typically used by farmers and plant and soil researchers for the measurement of this soil characteristic.
The parameter measured by a tensiometer is provided in units of centibars (cb) and millibars (mb) and is known as the ‘soil matric potential’ (ψm). In recent years the metric equivalent (S. I. units) of kilopascals (kPa) has also been adopted as a measurement unit for soil matric potential. The soil matric potential indicates the degree to which the plant needs to create an energy differential in its root system so as to initiate moisture migration from the soil into its root system.
Tensiometers provide their measurement, as mentioned, in centibars where 100 centibars equals one bar. The higher the centibar reading, for example 40 cb, the harder it is for the plant root system to extract moisture from the surrounding soil and conversely the lower reading, for example 10 cb, the easier it is.
Tensiometers are relatively simple devices comprising a hollow tube with a water porous but air impermeable ceramic tip (specified to a certain air entry value, say 1 bar) forming the lower end of an elongate water impervious tube. The tube is filled with water and sealed with a top cap. A vacuum gauge is attached to the top cap and the small volume of air above the water in the tube is used to measure the pressure inside the tube. The tube is then buried in a prepared hole in the soil with the ceramic tip in close contact with the soil at a desired depth within the soil profile.
When the matric potential of the soil is lower (more negative) than the equivalent pressure inside the tensiometer, tip which occurs when plant roots draw water from the soil surrounding the ceramic tip of the tensiometer, water moves from the tensiometer along a potential energy gradient to the soil through its saturated porous cup. This action creates suction or negative tension, which is sensed by the pressure gauge as an increase in negative pressure (vacuum) in the top part of the tube. Water flow into the soil continues until equilibrium is reached and the suction outside the tensiometer equals the soil matric potential. After rainfall or irrigation events, pore spaces within the soil fill with water. Water will then migrate back through the ceramic tip into the tensiometer tube due to now lower matric forces, which in turn lowers the negative pressure (vacuum) of the measurement instrument.
The exchange of water between the tensiometer and the soil it is located within works only to a specified negative pressure (suction, matric potential) of the ceramic tip. A ceramic tip with an air entry value of 80 cb experiencing a matric potential of 100 cb will allow air to migrate into the tensiometer tube. As soon as this occurs, the negative pressure of the instrument will rapidly drop to a value close to 0 cb, and the measurement of the instrument will not reflect the real prevailing matric potential of 100 cb and hence the instrument is not useable in such conditions. To rectify this situation, the tensiometer tube has to be re-filled with water. A hand vacuum pump attached to the top of the tensiometer tube has to be operated to draw soil water through the ceramic tip of the tensiometer to purge any trapped air bubbles within the ceramic tip with water. This corrective service will again establish the exchange of water only (in and out) between soil and the tensiometer tube water reservoir so that proper measurements can be taken by the tensiometer.
Installation of tensiometers according to the following guidelines is typical:                Installed shortly after plant emergence, between healthy average sized plants in a site that represents a soil type average for the planted crop.        One tensiometer is commonly installed at a depth of maximum root density, say 30 cm and another may be placed near the bottom of the active rootzone, say 60 cm, making up one profile measurement station. These depths may vary according to crop type.        One or two stations per crop of the same age and variety are commonly used. Other stations may be used to reflect changes in field topography. Readings should be taken at the same time each day preferably in the morning, noting that diurnal fluctuation in soil matric potential may require more than one reading per day.        At minimum readings should be taken every two days in medium textured soils and every day in light textured soils.        It is advisable to log tensiometer reading at a 30-minute time interval on a continuous basis so as to generate sufficient data upon which to base irrigation management decisions. Loggable tensiometers are fitted with a pressure transducer instead of a manual pressure gauge.        
Unfortunately, tensiometers have a variety of problems, including those described earlier and operate within a limited range of measurement, at best 0 cb to 80 cb. Tensiometers require specialised set up procedures and include the use of degassed water. Air tends to permeate the ceramic of the sensor especially when soil tension exceeds the air entry value. In sub-zero temperatures the water in the apparatus freezes rendering it inoperable. The device is fragile and requires continual maintenance despite its simple construction. The type of maintenance includes topping up with degassed water, vacuum pumping to purge air from the device and the need to add algacide periodically to the water in the tube so as to prevent algae build up.
Furthermore a tensiometer itself attracts plant roots in its immediate vicinity because it is a constant source of moisture. The latter of the abovementioned problems can skew the relevance of the measurements taken, as the field measurement point of the tensiometer may now have a significantly larger root length density per soil volume than the surrounding crop.
A tensiometer is limited in its ability to determine matric potential, which must lie between 0 kPa (saturation), and 80 kPa (dry). This is due mainly to the fact that the vacuum gauge or manometer measures a partial vacuum relative to the external atmospheric pressure and with the general failure of water columns in macroscopic systems to withstand tensions exceeding 1 atmosphere or 1 bar or 100 kpa. Some agricultural crops are subjected to a managed soil moisture condition exceeding this threshold and hence tensiometers cannot be reliably used for these applications.
Matric potential is described herein as a force per unit area or pressure which has units of pascals or mega-pascals (1 Mpa=10 bars, 100 kPa=1 bar). One atmosphere is equivalent to the pressure exerted by a column of water 100 cm high.
There are also reading range problems associated with soil texture. For example, the hydraulic contact between a coarse sand matrix and the tensiometer tip is more easily interrupted than in heavy textured soils (below 85 kPa).
A tensiometer is not depth compensated. The tensiometer equation is:ψm=ψgauge+(zgauge−ztip)The vertical distance from the gauge plane zgauge to the tip ztip must be added to the matric potential measured by the gauge (expressed as a negative quantity) to obtain the matric potential at the depth of the tip. This accounts for the positive head at the depth of the ceramic tip exerted by the overlaying tensiometer water column. For example: −3 cb needs to be added for each 30 cm of instrument length.
The accuracy of the tensiometer is no better than +/−1.2 kPa in wet conditions. A higher resolution at the “wet end” would allow the tension measurement technology to be applicable to other markets such as nurseries and greenhouses. Container media consisting mostly of materials such as peat, bark, sand, perlite and vermiculite are combined to form a mix, which holds a large percentage of volumetric water content. However a matric potential measurement of 8–10 kPa indicates that the mixture is a relatively dry medium.
Tensiometers cannot be used immediately after installation. The sensing tip has to be soaked in degassed water for a couple of days. Letting the tip dry out renders the instrument useless for measurement until rewatered.
Owing to the hydraulic resistance of the tensiometer tip (cup) and the surrounding soil and the contact zone between tip and soil, the tensiometer response of some instruments may lag behind suction changes in the soil, ie there exists a finite response time.
Tensiometers are not frost tolerant. If water freezes inside the tube, the expansion caused through ice formation can potentially destroy the instrument.
Installation of the instrument can cause problems, especially deep installation where the tensiometer tip has to be pushed into a slightly undersized hole at the bottom of a larger access hole. The tip can easily break off while being pushed into the lower and unseen undersized hole.
Air bubbles frequently appear inside the water-filled tube connecting the porous tip to the manometer. This occurs because of the reduced solubility of gases at lower hydrostatic pressure (as well as higher temperatures) and also because of the diffusion of gases from the air phase of the unsaturated soil trough the porous walls of the tensiometer tip. Occurrences of these bubbles do not immediately negate the measurement but reduces its sensitivity.
Therefore tensiometers require continual labour intensive maintenance. There is an ongoing risk of not being able to take readings from the Tensiometer because the instrument has ceased to work due to lack of adequate maintenance.
One tensiometer providing one measurement point is not very useful to indicate soil moisture profile dynamics throughout the crop's rootzone. It is common practice to use 3 to 4 individual tensiometers to present measurement points throughout the soil profile, which can triple or quadruple the already intensive manual maintenance requirements.
Tensiometers are unable to measure rising and falling levels of salinity, since the wall of the tensiometer' porous cup is permeable to both water and solutes. Solutes in the soil solution diffuse freely into the cup so that the water inside the tensiometer tends to assume the same solute composition and concentration (osmotic potential) as the surrounding soil water.
Instrument precision and accuracy to measure matric potential for the purpose of irrigation scheduling of commercial crops is critically important to a crop grower, as the economic return of their enterprise depends on the quality of these measurements.
Furthermore is the knowledge that at a particular soil water tension the crop will be going into water stress. Clearly the ultimate aim is to provide appropriate crop management information based on accurate tensiometer readings, as these will be used to make critical irrigation management decisions.
Not only are the water contents and its associated matric potential important for irrigation scheduling but so is the management of fertilizer concentration in the soil. More particularly it is of primary importance that there be an availability of beneficial solutes in the soil water (as provided by fertilizer) at a depth that is in the active uptake zone of the crop root system. Fertilizer salts are easily leached from the rootzone in humid environments through rainfall or in arid environments through over irrigation practice.
Harmful salt concentrations can built up naturally in surface soils of arid and semi-arid regions through weathering of rocks and minerals and insufficient rainfall to flush these salts from the upper soil layers. Under-irrigation practices, where irrigation water salts and fertilizer salts accumulate in the rootzone will also potentially become damaging to crop health.
In poorly drained soils downward movement of the irrigation water and fertilizer salts to the groundwater is impaired, leaving salts in the soil to be brought up later to the surface as the irrigation water evaporates. A saline soil is thus created. Historically, large increases in worldwide food crop requirements have been satisfied by expanded irrigation practices. However, in many areas the need for good drainage was overlooked and the process of salinization has been accelerated.
Salt concentrations can also be concentrated in the rootzone through rising watertables or horizontally moving fossil salt deposits moving over impervious geological layers. These may ultimately rise to the soil surface in the low-lying parts of the landscape forming saline seeps.
In arid areas of the world where evaporation is high, the salt build-up in irrigated soils must be monitored to ensure successful plant production and to stop fertilizer leakage into waterways.
Soil salinity and the inseparable moisture content can be used to determine fertilizer migration dynamics within a soil profile.
It was in recognition of the described shortcomings of current methods of soil moisture and salinity measurements that the following device and installation procedure is proposed.
In summary, the inventors have developed a method and means capable of measuring and tracking the soil matric potential and soil salinity in a diverse range of soil types.
The apparatus and method aims to provide advantages over existing technology with one or more of the following attributes:    1. Increased accuracy and precision    2. Increased measurement range    3. Increased measurement resolution    4. Useable in range of soil types    5. No need for calibration (uses one factory calibration and a unique data processing model)    6. Able to monitor almost continuously    7. Instant useability    8. Reduced lag time    9. Simplified Installation    10. Sensor to be maintenance free    11. Depth compensating (no calibration)    12. Sensor to be frost proof    13. Profile measurement approach    14. Does not attract roots that may adversely affect or skew recorded data    15. Measures soil solution salinity simultaneously with soil moisture    16. Measures soil solution salinity (or pore water salinity) simultaneously with soil matric potential    17. Measures pore water salinity that is corrected for changing soil moisture conditions (matric potential)    18. No site destruction if sensor needs to be replaced
To remove the need to calibrate a particular soil salinity sensor to measure soil salinity in hundreds of different soil types, the invention in a preferred arrangement provides for only one specific sensor calibration equation. This calibration equation is applicable to all manufactured sensors after sensor normalisation. The sensor is calibrated for matric potential and soil solution salinity (pore water salinity) in the specific media only and not for the surrounding soil with which the media maintains hydraulic contact and moisture exchange.
The inventors described herein is a method and means for measuring the soil water content and soil salinity in a single medium for which the determination of soil matric potential is merely a look up table exercise. The combination of sensor and medium is calibrated once and holds true for the period of use of the medium.
This approach is reliant on the medium having the characteristic of allowing the available soil moisture solutes to migrate into and out of the surrounding soil.
Furthermore, it is important that the field of influence of the sensor for measuring soil moisture and salinity is located wholly within the specified media. For sensors that use electromagnetic techniques that are used to determine soil moisture and salinity, the field of influence of the radiated electromagnetic radiation preferably remains totally within the volume of the media.
The use of a specified media guarantees that the measurements taken by the sensor and the data processing model, calibrated uniquely for the specified media, provide an accurate value of the soil matric potential of the media. According to the model, the soil in hydraulic contact with it has the same matric potential. Furthermore, the volumetric water content of the media, and hence, the salinity of the soil solution entering the media can be simultaneously measured.
The inventors have developed an arrangement of elements that, when used in a predetermined manner provide a way of measuring the matric potential and the associated measured volumetric soil water content of the medium (to derive matric potential) as well as the salinity of the soil solution entering the medium.
The clear advantage of such an apparatus and method is that only one combination of elements (elements being sensors, media and data processing model) need to be calibrated. That calibration is done in the laboratory, on the type of medium to be used and the resultant calibration equation is then relevant to all sensors produced (for that particular physical construction). The medium is arranged so that its moisture migration characteristics make the model useable in most soil types. Further more this apparatus and arrangement relies not upon sensing matric potential using the tensiometer principle but by sensing the total volumetric soil water content of the medium using a maintenance free and measurement range insensitive electromagnetic sensor. The calculation of the matric potential of the soil is done using a previously derived soil water release curve (calibration equation) derived for the media.
Most usefully, the apparatus does not need adjustments or corrections of any type between usage in a wide variety of different soil types. At the site, the pre-calibrated apparatus, including the predetermined media is merely installed. Due to automated measurement and data collection techniques, useful data for presentation to the user can be collected within an appropriate time and without any undue lag caused by the measurement devices operation. The measurement results are provided in well-recognized units, namely matric potential in kilopascals (kPa) and soil solution salinity in milli-Siemens per centimeter (mS cm−1).